Known for flow measuring devices are sensors, which work according to the thermal measuring principle. This measuring principle is based on the cooling of a heated resistance thermometer, subsequently referred to as the heated sensor element, from which heat is withdrawn by the flowing measured medium. The withdrawn energy is compensated by increasing the electrical heating current. In this way, a constant temperature difference between the heated sensor and a reference sensor is established. The reference sensor is referred to herein as the measuring sensor element. The greater the mass flow, the more energy is required, in order to maintain this difference in temperature. The measured heating current is, as a result thereof, proportional to the mass flow. The thermal measuring principle is well established in processes, in which a product is made from a raw or starting material by the application of chemical, physical or biological procedures and is applied in numerous applications successfully. Especially demanding is the application of the measuring principle in water, since in such case the heat transfers and the required heating power are highest. In the case of small separation of the sensor elements from one another, crosstalk of the heated sensor element to the temperature measuring sensor element can be experienced, especially in the water flow velocity range above 2.5 m/s.
There are sensors known, which are composed of two sensor elements, a heated sensor element and a measuring sensor element, each of which has a cylindrical sensor cap and extends from a base surface of a sensor. Resistance sensors are soldered in the end faces of the sensor caps. Due to the cylindrical sensor caps, the sensor elements already have a good thermal insulation. These sensors, however, behave poorly in water, as compared with the earlier mentioned sensors. Thus, their characteristic curve exhibits in the case of rising flow a degree of saturation, such that, in spite of higher velocity, no more heat can be expelled into the flow. In such case, the Nusselt number, which describes the heat transfer as a dimensionless number, is too high.
Published International Application, WO 2012/012769 discloses a sensor, which has a cast-on encasement, which extends to the ends of two metal sleeves. These metal sleeves are, however, full surfaced, thus also heated in the region of their lateral surfaces. The encasement serves in this connection for shielding a part of the heating surface.